Metal processing



United States Pfltefif Q re METAIsr PROCESSING Hiiglj" G. Webster and Robert" H. Slioeiiiaker,-. Detroit,

8 Claims. oi? 2114 1 15) Thisi invention relatesato improved \heat treating? and electrolytic cleaning ,ofarmetals 'to-v remove: surface purities such as oxides and scales usually associated therewith by placing the meter-warn piece in a molten salt type of bath andsubjectingIsame to electrolysislinwhich themetal work piecei stalternately. electrolized withdimeet current; fir'st'as a" cathodetand then-as an anode:

The metal-"work piecef is usually of ferrous metal, but

itniay. be of non-ferrous-ametaln An:outstanding;bath

forq'cleaning of metal 1 ofl thisacharacteris known in the art, as th'eTK 4 bath, and is: particularly: described-in prior-Patent No. 2,468,0il5l T hatb ath preferably comisons-90% o'f caustic-neither sodium or potassium hydroiiideaiid smallipropo'rtions,such-as 7+-13%-of sodium chloride and l5 '5f%@ oflsodi um aluminate, all" proportions by weight When the metal work piece contains sand orsili'ca inclusions inathe surface usually resultingfrom casting,-..the bath: may, further contain 2'-6% of alkali metalifluoride, Whileathanbath has hadgreat commercial. successin the cleaning of metal as described inlthat patent, for purposes-0fthe presentinvention the electrolytic cleaning; bath may l be further varied 2 to contain minor contents, -15% of alkali metal borate, 1-15% of alkali metal fluorob'or'ate, 1-10% of alkali metahcyanide, 1-15% of alkali metal flue-silicate or 1'-1 0% ofalkali metal silicate'. V

It is 'usuallyfound desirableito preheat metal, articularly ferrous metal" at a" temperatnre rangingj'from 1100 to"1 35 0": F.,v usually around 1200*" F1, forpurposes' of stress relieving the metal. That priin'ary' heattreatmentfor stress-relieving"according tothe present invention, is-

performedin' a molt'eulsaltba't h; primarilybecause it is possible "by thatmoltenlsalfbatli heat tr'eatmentto secureoptiriirin'i' heatti'ans'fefto all p'arts'of the metal vand because it is possible in such bath to obtain an exact terriperature control at which themetal is heated to obtain optimum istress relief.

The electrolytic treatm'e'ntin asalt bath, as described aboveylgives iby farthe-bestcleanin'g arid-'- descaling of the metal surface; The cleaning 'operation by electrolysis inv such molten salt bath, howevr,-is usually ata con siderably lower temperature'possibly applied in'the range from 600 to 1000 F.', but prefefably'from 850 to 900 F Accordingly, it isnotpractically possible to simultaneously use' the' same electrolytic cleaning bath' for"heating'the metal to stress relieve-it, before application of electrolysis toclean' it.

Where metal' is cleaned I in'ce'rtain known molten salt baths"wliose'- composition 'is fiindai'neritally different from that used for electrolytic cleaning, there maybe an attack upon the metal oxides and sc'ale'on the metal, without electrolysis, particularly, when the" bath contains an oxidizing agent like a nitrate-salt. Such bath is inferior to' an electrolytic bath as' described above, for cleaning' the metal, but more important, such'bath will both corrode the bath'apparatusat the higher stress-relieving ternperatureand often convert the oxide and scale on the 2,847,374 Patented Aug. 12, 1 8

2 metal to a form rendering them even more-difficult to remove 'byelectrolysis in an electrolytic cleaning-bath of the character mentioned above. Moreover, iffsiich varied composition sti'ess relieving' bath were used, it wou'ld" necessitate, after removal of the metal frornithefstress' relievingbath, co'olingthe metal su'flicient' to'wash"the adhered molten or solidified saltsfro'm the surface nd drying of the metal before immersion in the electrolytic" cleaning bath; On the other hand,'. if the metal were,

taken from the stress relieving bath, cooled with -the" molten salt thereon and passed directlyinto the'ele'ctio lytic'cleaning bath, the cleaning bathw'ould soon become so contaminated with undesirablesalt com onents'carned' over from the stress relieving'b'ath that the electrolytic cleaning? operation would become considerably?"inferior and, in a'short'time, the electrolytic cleahing'bath would be rendered useless" by the" accumulation of' such con taminating materials;

It is a'prima'ry object of the present invention: to provide' a stress-relieving bath which,.without electrolysisin the stress-relieving heat treatmenn does not adverselyl affect the adherent surface impurities uponthe metal such as oxidesand scale. V I

' It is a'further object to form such bath of moltensaltmaterials capable both: of functioning :satis-factorilyfor stress-relief and which, upon subsequent transfenfrom-the" stress-relieving bath without removal of-molten salt-thereonto the: electrolytic cleani-ng.-'bath,-,will notsubstanti-ally contaminate thecleaning-tbath or in any- Wiseinterfere with -the efficient electrolytic-cleaning operation.

This inventionis based in part-upon the disc0very=thatmolten salt baths of the type-described: above,--Which':are useful forcleaning ofmetals, particularly'ferrous metal,-

electrolytically, will not, in the absence of appliedlelectrol ysis 'to such molten salts adverselyaffect the-oxide or scale upon the metal, whereby such molten: SaltS,':1lS 6fl1l"' for electrolytic cleaning, are also-eminently suitable ma-- terials for a stress-relieving bath; Furthermore, when metal is stress-relieved in'suchbath materials by subjectinglto a temperature-in the above mentioned stress reliev-- ing range;.upon-removal from such bath after heat treat ing,-the metal does notneed toh'avethe adherent-moltensalts-removed, since they may be of the same character as the bath used for electrolytic cleanin'gi Thus, the-' metal, after heating-for stress-relief and-removal from"- that molten saltbath: coated with adherent molten salts, may be directly placed in another bath of substantially the-same molten s'altmaterial-at a low'eftemperature and electrolyzed to effect the cleaning-ithereof in a manner known-in the-- art, for instance, as described in patent above identified;

It-will be appreciatedthat'usingvthe-same' ba'th mate= rials, 'for both baths, the first bath forstress relieving- Withoutelectrolysis and thesecond-bath subsequentlyfdr electrolyzing' to clean the metal, the'transfer' of molten salt 'from'one bath to' the other does *not' tend to adversely 'vary the composition'of the cleaning bath. "Moreover,

use 'of the electrolytic type of bath for stress-'relieving"tlie oxide and scale coating upon the metal is not afiecte'd m' a' into the molten salt stress-relieving bath of the character described, water Vapors, i. e., steam, preheated to a temperature that will not substantially affect the temperature of the bath. Alternatively, it is found that addition of certain hydrated salts which will decompose and release Water vapor 'at the high temperatures involved will overcome thecorrosive effect upon the stress-relieving apparatus.

That hydrated salt should be of an alkali or alkali earth metal. For instance, it may be one of the salts normally forming the bath of the type above mentioned, which contains water of crystallization, for instance, a hydrated alkali-forming metal borate or a hydrated caustic alkali, but is preferably a hydrated alkali earth salt which does not lose its water of hydration until subjected to high temperatures to melt it or dissolve it in the molten salts of the bath. For this purpose alkali earth metal hydroxides which are hydrated and tend to bond the water of hydration without release until relatively high temperatures are reached, may be used.

The various alkali earth metal hydroxides such as barium, calcium or strontium hydroxides containing water of crystallization may be used; and of this group, barium hydroxide is preferred because it does not appear to adversely affect the fluidity as well as other desirable properties of the bath, and particularly, as it accumulates in the subsequent cleaning bath does not adversely affect the operation thereof in contrast to other diluents carried over from the stress-relieving bath which are often objectionable.

Accordingly, it is a further object of this invention to impart a water vapor content to the stress-relieving bath to inhibit corrosion either by way of addition of steam directly to the bath at about the temperature of the bath, or by addition of hydrated salts which release water vapor in the molten salt bath. As additive salts, preferably we use hydrated salts to supply water vapor to the stress-relieving bath of the same kind as appear in the electrolytic cleaning bath, except that alkali earth metal hydrate, preferably barium hydroxide, may be used in the stress-relieving bath in the form of their hydrates for supply of water vapor thereto.

Where an alkali earth metal hydrate is added to the stress-relieving bath it will be present in proportion of about 1 to 15% for humidification thereof. Where the humidification is effected by passing steam into the stressrelieving bath, it may be added as a continuous supply of steam bubbled into the bath at the rate of about 1 to 100 lbs. per hour, per 1000 lbs. or molten bath salts; or the bath may be treated with steam periodically by supplying l to lbs. of steam passed into the bath over a period of /2 to 2 hours every 5 to 24 hours, as a series a of intermittent applications.

It will be understood that the treatment of the stressrelieving bath with steam or addition thereto of salts capable of releasing Water of crystallization is for the purpose of offsetting the corrosive effects of the stress relieving salts upon the apparatus containing the molten salt stress-relieving bath. Where that specific function to prevent corrosion is not needed for the specific apparatus, that type of treatment may be omitted.

The following examples illustrate the practice of this invention:

Example I Two baths, both having the same composition, consisting of 85% sodium hydroxide, sodium chloride, 4% sodium fluoride and 1% sodium aluminate, by weight, are made up in separate tanks by heating the composition above the melting point thereof. The tank for cleaning the metal may have the same construction as shown in prior application, Ser. No. 239,934, now Patent No. 2,732,339. The tank for stress-relieving may be of the same construction, except that electrodes are omitted. The stress-relieving tank is heated to a temperature of 1200 F. The electrolytic cleaning bath is heated to 875 F. Gray iron castings are first placed in the stress-relieving tank and held therein for one hour, thereby being heated at 1200 F., in the molten salt composition. The castings are then removed from the stressrelieving tank and transferred to the cleaning tank wherein they are made first a cathode and current is passed therethrough for a period of 20 minutes at a volatge drop of 2% to 6 volts, using 1000 amperescurrent for each 4000 lbs. of bath composition. Thereafter, thecnrrent Example [I The procedure for stress-relieving and cleaning metal as set forth in Example I is repeated, except that a molten salt bath and stress-relieving composition, the same composition for both'baths, is substituted asfollows: 81% of sodium hydroxide, 5% sodium tetraborate decahydrate (borax), 4% sodium fluoride and 10% sodium chloride. It was found that the bath substance of this example showed no tendency to corrode the stress-relieving bath over a period of 5 days of continuous use, whereas that of Example I showed signs of incipient corrosion of the tank walls of the stress-relieving bath. However,

upon continued use of the bath of this example for 2 weeks, the stress-relieving bath too began to show signs of corrosion thereby indicating that the capacity to evolve water vapor for protection of the tank is exhausted after about 5 days of use. The metal was stress-relievedas well as in Example I and the surface was acceptably cleaned, but not quite as clean as in Example I.

Example Ill Metal was first stress-relievedand then cleaned in the I same bath compositions and under the same operating conditions as described in Example I, except that in the stress-relieving bath containing 2500 lbs. of fusedsalt, high temperature steam at about 1200 F. was continuously bubbled into the bottom of the fused bath at' the rate of approximately 5 lbs. of steam p'er hour. It was found after an operating period of 2 weeks that there were no signs of corrosion on the stress-relievingv bath apparatus.

Example IV The same cleaning bath as described in Example I was used in combination with a stress-relieving bath having the composition of sodium hydroxide, 10% sodium chloride, 4% sodium fluoride, 1% sodium aluminate and 5% barium hydroxide octahydrate. It was found that the metal was cleaned equally well as in Example I, there being no apparent deteriorating efiect upon the electrolytic cleaning bath by bariumhydroxide transferred from the stress-relieving bath. 'It was further found that there were no signs of corrosion upon the stress-relieving apparatu until about 12 days of use of this bath, thereby indicating that the hydrated barium salt has a substantial effect to inhibit corrosion which is substantially superior to that of borax. 1

Asvthus described, stress-relieving of metals as a primary step applied prior to cleaning of the metal is desirably effected in a bath having either the same salt composition as that of the electrolytic bath or one having hydrated components of the same composition as the electrolytic bath or having hydrated components of, alkali earth metal, salts whose anions correspond totho se al; ready present in the'electrolytic cleaning bath.

'versely affect oxides and scale upon the metal to be stress-relieved whereby the metal is subsequently efiiciently cleaned in an electrolytic cleaning bath of the molten salt type.

Moreover, in another aspect of this invention, it is found that if the electrolytic bath is humidified by passing therein continuously or intermittently small quantities of water vapor, substantial corrosion of the stressrelieving apparatus may be inhibited. That same humidi fication may be efiected by using, in the stress-relieving bath, salts normally useful in the electrolytic cleaning bath, one or more of which are desirably in the hydrated form to contain water of crystallization. Where an hydrated salt is used to supply water other than a hydrated salt of the type used in the electrolytic cleaning bath for humidification of the stress-relieving bath, that salt may be an alkali earth metal salt having an anion corresponding to those present in the electrolytic cleaning bath, preferably a hydrated hydroxide of an alkali earth metal, and barium hydroxide of this group, is outstanding for this purpose.

We claim:

1. The method of cleaning and stress-relieving ferrous metal having adherent surface impurities of the character of oxides and scale comprising first heat treating the metal in a non-electrolytic stress-relieving bath at a temperature of about 1100 to 1350 F., and then transferring the stress-relieved metal coated with salts of the stressrelieving bath to an electrolytic cleaning bath of the molten salt type maintained at a temperature in the range of about 600 to 1000 F. and cleaning the metal therein electrolytically, the molten salt composition of both baths comprising about 75 to 90% of caustic alkali, up to about 6 15% of sodium chloride, up to about 6% of alkali metal fluoride, up to about 10% alkali metal cyanide, 0 to 10% alkali metal silicate and the remainder, at least 0.5%, selected from the group consisting of sodium aluminate and alkali metal borate.

2. The method as defined in claim 1, wherein the molten salt stressreliev-ing bath is humidified by addition of water vapor thereto.

3. The method as defined in claim 1, wherein the molten salt stress-relieving bath is humidified by having one of its salt components in hydrated form.

-4. The method as defined in claim 1, wherein the molten salt stress-relieving bath has added thereto 1 to 15 of a hydrated alkali earth metal hydroxide.

5. The method as defined in claim 1, wherein the molten salt stress-relieving bath has added thereto 1 to 15% of hydrated barium hydroxide.

6. A stress-relieving bath composition in which the metal parts are heated for stress-relieving, comprising by weight, about to sodium hydroxide, 7 to 13% sodium chloride, up to about 6% sodium fluoride, 0.5 to 5% sodium aluminate and 1 to 15 of a hydrated hydroxide of an alkali earth metal.

7. A stress-relieving molten salt bath composition in which metal parts are heated for stress-relieving comprising about 75 to 90% sodium hydroxide, 7 to 15% sodium chloride, up to about 6% sodium fluoride, 0.5 to 5% sodium aluminate and 1 to 15% hydrated barium hydroxide.

8. A stress relieving molten salt bath composition in which metal parts are heated for stress relieving, comprising about 75 to 90% sodium hydroxide, 7 to 15% sodium chloride, 0.5 to 5% sodium aluminate and l to 15 of hydrated barium hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,458,663 Webster Jan. 11, 1949 2,468,006 Webster Apr. 19, 1949 2,738,294 Spence Mar. 13, 1956 

6. A STRESS-RELIEVING BATH COMPOSITON IN WHICH THE METAL PARTS ARE HEATED FOR STRESS-RELEIVING, COMPRISING BY WEIGHT, ABOUT 75 TO 90% SODIUM HYDROXIDE, 7 TO 13% SODIUM CHLORIDE, UP TO ABOUT 6% SODIUM FLUORIDE, 0.5 TO 5% SODIUM ALUMINATE AND 1 TO 1K% OF HYDRATED HYDROXIDE OF AN ALKALI EARTH METAL. 